1. Field of the Invention
The present disclosure relates to the backlight unit. Particularlyy, the present disclosure relates to the backlight unit for preventing the optical sheet from cracking and the liquid crystal display (or “LCD”) using the backlight unit.
2. Discussion of the Related Art
Nowadays, due to the characteristics of light weight, slimness, low electic power consumption, the liquid crystal display device is being widely more and more applied. The liquid crystal display device is used in portable computers such as a notebook PC, official automation devices, audio/video devices and external/internal advertizing display devices. The transparent type liquid crystal display device, the type most commonly used, shows the picture data by modulating the luminescence of the light incident from the backlight unit by controlling the electric field applied to the liquid crystal layer.
As the LCD device is not a self-luminescence display device, it requires a light source such as s backlight Unit. There are two types for the backlight units for LCD devices; the direct type and the edge type. For the edge type, the light source is disposed around the flat panel and the light from the light source is guided to the front surface of the LCD panel using a transparent light guide. For the direct type, the light source is dispose on the rear surface of the LCD panel so that the light from the backlight source is directly radiated to the LCD panel. Compared with the edge type, the direct type can have a brighter luminescence by using more light sources. Further, the direct type has an advantage in making the light irradiating surface larger. Therefore, for the LCD TV requiring a large size LCD panel, the direct type is generally used.
The direct type backlight unit comprises a light source 10 irradiating light to the liquid crystal display panel, a bottom cover 20 housing the lighit source 10, a support side 30 supporting the light source 10 at both open sides of the bottom cover 20, and diffusion plate 40 and optical sheets 50 sequentially stacked on the light source 10.
The bottom cover 20 includes a bottom surface 20a, slant surfaces 20b at both sides of the bottom surface 20a, and upper sufaces 20c expanded from each slant suface 20b to stack the diffusion plate 40 and optical sheets 50 thereon. At the upper surfaces 20c of the bottom cover 20, a first protrusion 22 is formed for preventing the diffusion plate 40 and optical sheets 50 from moving freely. In the interim, on the upper surface 32 of the support side 30, a second protrusion 34 is formed for preventing the diffusion plate 40 and optical sheets 50 from moving freely.
The diffusion plate 40 diffuses the light incident from the light source 10 to the front direction of the liquid crystal display panel to make the distribution density of the light irradiating to the liquid crystal display device to be uniform. The diffusion plate 40 is guided by the first protrusion 22 and the second protrusion 34 so that it is fixed on the upper surface 20c of the bottom cover 20 and the upper surface 32 of the support side 30.
The optical sheets 50 make the transmitting angle of the light diffused by the diffusion plate 40 to be vertical to the liquid crystal display panel to increase the light transmitting efficiency. The optical sheets 50 include at least one diffusion sheet and prism sheet. The optical sheets 50 include a first hook 52 having a first elliptical hole 52a and a second hook 54 having a second elliptical hole 54a so that they are fixed on the upper surface 20c of the bottom cover 20 and the upper surface 32 of the support side 30. The first elliptical hole 52a formed at the first hook is engaged with the first protrusion 22 formed at the upper surface 20c of the bottom cover 20. The second elliptical hole 54a formed at the second hook 54 is engaged with the second protrusion 34 formed at the upper surface 32 of the support side 30. Here, the first and the second elliptical holes 52a and 54a make point contacts or contacts with narrow contact surface to the first and second protrusions 22 and 34, respectively.
During deliverying the liquid crystal display device havin the backlight unit, if an impact force (ST) is applied to the LCD device to the −y direction from outside, the impact force (ST) causes concentrated stress on the point (P1) where the line part (A1) of the first and second elliptical holes 52a and 54a meets to the curved part (A2), to the +y direction, as shown in FIG. 3a. Due to the concentrated stress, certain point (P1) of the optical sheets 50 may be cracked as shown in FIG. 3b. The first and second hooks 52 and 54 of the optical sheets 50 week to the impact force applied to the +y and/or −y directions. Sprcifically, the occurrence ratio of the crack problem is increased because the thickness (t) of the rib portion is thinner due to the restriction of the bezel width, and the the endplay is increasing at impact due to the large LCD device size.
In the currently manufactured LCD device, there is no structural element for absorbing the impact force concentrated to the hook of the sheel, when impact force is applied to the backlight unit from the outside. Furthermore, the liquid crystal display device having a backlight unit according to conventional art may have the second problem due to the impact force such that the sheet may be taken off, wrinkeled, or dented as well as the display quality may be degraded due to the intrusion of the foreign materials generated due to cracks in the sheet.